ArticulationController questions

[dan.sandberg]

Does the ArticulationController use a drive (PositionTarget) if it is present or does it only use JointState?

Are the names in the URDF mapped to the names in the USD or does it instead assume the joints are in some specific order?

I’m asking cause I have a robot that doesn’t follow a target properly and I’m not sure how to debug it. I’m using the LulaKinematicsSolver since it is simpler than the RMP stuff.

Thanks!

[bpeele]

Hi Dan,

I recommend using the script editor to make sure the Articulation is working as expected.

I will show an example for the UR10, but the same thing can be done for any robot in the scene.

First I add a UR10 to the scene using “Create → Isaac → Robots → Universal Robots → UR10”

Then I press play to start the simulation and open the script window using “Window → Script Editor”

With the simulation running, you can then use a little script to see the names of each joint in the articulation, set target positions and read joint positions:

from omni.isaac.core.articulations import Articulation, ArticulationSubset
from omni.isaac.core.utils.types import ArticulationAction
import numpy as np

# Load robot
robot = Articulation("/UR10")
robot.initialize()
print("DOF names:", robot.dof_names)

# Set position for all joints
robot.apply_action(ArticulationAction(np.array([1.0, 0.0,2.0,3.0,0.0,0.0])))

# Print position
position = robot.get_joint_positions()
print("position:", position)

Here is my expected output from running it twice. Note the first time the position readout is near zero because the robot hasn’t had time to move directly after applying action in the same step. The second set of output very closely approximates the target position.

DOF names: ['shoulder_pan_joint', 'shoulder_lift_joint', 'elbow_joint', 'wrist_1_joint', 'wrist_2_joint', 'wrist_3_joint']
position: [ 1.2860776e-06  7.5394190e-03  1.6797542e-03 -1.4944457e-06
  1.0166268e-07 -6.6924440e-07]
DOF names: ['shoulder_pan_joint', 'shoulder_lift_joint', 'elbow_joint', 'wrist_1_joint', 'wrist_2_joint', 'wrist_3_joint']
position: [ 9.9996537e-01  3.5953731e-03  1.9993634e+00  3.0001130e+00
 -6.8680338e-06  3.4853120e-09]

[dan.sandberg]

I ended up getting things working by using the forward-kinematics in Lula to show where Isaac Sim thought each joint was. Then I changed things in OnShape so that the origin of each part was at the joint. Eventually by repeating this for each joint I ended up with a URDF that would work. I also needed to add “limit” to the prismatic-joint because otherwise Lula crashed.

Took I think a week of effort (changing OnShape model, hand-editing the URDF) to get the URDF to work properly in Isaac Sim.

In any case, what about the question below – Does the articulation need a driver? Does it use the “target” if a joint driver is present? Does it use the joint value directly if there is no joint driver?

@dan.sandberg

Does the ArticulationController use a drive (PositionTarget) if it is present or does it only use JointState?

[bpeele]

The recommended process for OnShape import is to add joint drives to each joint.

This tutorial describes the process: Onshape importer — Omniverse Extensions latest documentation (search “Add Joint Drives”)

A video tutorial is available here: https://www.youtube.com/watch?v=gYVzaTAgHvQ

The joint drives will about the Articulation interface to be used to set position targets as I showed in the previous snippet.